Measure the length of the room using your tape measure.
Measure the width of the room using your tape measure.
Multiply your room's width by your room's length, which gives you your room's square footage.
Consult the infrared heater's user guide to obtain the British thermal unit, or BTU, specifications.
Match the infrared heater's BTU to your room's square footage, knowing that it requires approximately 45 BTU to successfully heat 1 square foot of space. For instance, if you have a 1,000 square foot home, you need a heater capable of producing 23 BTU, because 23 multiplied by 45 equals 1045.